Transposable elements in Drosophila montana from harsh cold environments.

Background: Substantial discoveries during the past century have revealed that transposable elements (TEs) can play a crucial role in genome evolution by affecting gene expression and inducing genetic rearrangements, among other molecular and structural effects. Yet, our knowledge on the role of TEs in adaptation to extreme climates is still at its infancy. The availability of long-read sequencing has opened up the possibility to identify and study potential functional effects of TEs with higher precision. In this work, we used Drosophila montana as a model for cold-adapted organisms to study the association between TEs and adaptation to harsh climates.

Results: Using the PacBio long-read sequencing technique, we de novo identified and manually curated TE sequences in five Drosophila montana genomes from eco-geographically distinct populations. We identified 489 new TE consensus sequences which represented 92% of the total TE consensus in D. montana. Overall, 11-13% of the D. montana genome is occupied by TEs, which as expected are non-randomly distributed across the genome. We identified five potentially active TE families, most of them from the retrotransposon class of TEs. Additionally, we found TEs present in the five analyzed genomes that were located nearby previously identified cold tolerant genes. Some of these TEs contain promoter elements and transcription binding sites. Finally, we detected TEs nearby fixed and polymorphic inversion breakpoints.

Conclusions: Our research revealed a significant number of newly identified TE consensus sequences in the genome of D. montana, suggesting that non-model species should be studied to get a comprehensive view of the TE repertoire in Drosophila species and beyond. Genome annotations with the new D. montana library allowed us to identify TEs located nearby cold tolerant genes, and present at high population frequencies, that contain regulatory regions and are thus good candidates to play a role in D. montana cold stress response. Finally, our annotations also allow us to identify for the first time TEs present in the breakpoints of three D. montana inversions.